This invention relates generally to golf ball compositions and methods for making golf balls from these compositions. The compositions are formulated to optimize the golf balls' performance properties.
Golf balls generally comprise a core and at least one cover layer surrounding the core. Balls can be classified as two-piece, multi-layer, or wound balls. Two-piece balls include a spherical inner core and an outer cover layer. Multi-layer balls include a core, a cover layer, and one or more intermediate layers. Wound balls include a core, a rubber thread wound under tension around the core to a desired diameter, and a cover layer, typically of balata material. Generally, two-piece balls have good ball distance when hit and durability, but poor “feel”—the overall sensation transmitted to the golfer while hitting the ball—and low spin rate, which results in poor ball control. Wound balls having balata covers generally have high spin rate, leading to good control, and good feel, but they have short distance and poor durability in comparison to two-piece balls. Multi-layer balls generally have performance characteristics between those of two-piece and wound balls; that is, multi-layer balls exhibit distance and durability inferior to two-piece balls but superior to wound balata balls, and they exhibit feel and spin rate inferior to wound balata balls but superior to two-piece balls.
Material characteristics of the compositions used in the core, cover, and any intermediate layers are among the important factors that determine the performance of the resulting golf balls. In particular, the composition of the core is important in determining the core's coefficient of restitution (C.O.R.) and compression ratios, which are important factors in determining the ball's speed. Further, the composition of intermediate layers in multi-layer balls is important in determining the ball's spin rate and controllability. Finally, the composition of the cover layer is important in determining the ball's durability, scuff resistance, speed, shear resistance, spin rate, “click” (the sound made by a golf club head when it hits the ball), and feel.
Various materials having different physical properties are used to make core, cover and intermediate layers to create a ball having the most desirable performance possible. For example, many modem cover and intermediate golf ball layers are made using soft or hard ionomeric resins, elastomeric resins, or blends of these. Ionomeric resins used generally are ionic copolymers of an olefin and a metal salt of a unsaturated carboxylic acid, or ionomeric terpolymers having a co-monomer within its structure. These resins vary in resiliency, flexural modulus, and hardness. Examples of these resins include those marketed under the name SURLYN manufactured by E.I. du Pont de Nemours Company of Wilmington, Del., and IOTEK manufactured by Exxon Mobil Corporation of Irving, Texas. Elastomeric resins used in golf balls include a variety of thermoplastic or thermoset elastomers available. Ball cores generally are made from polybutadiene rubbers.
Each of the materials discussed above has particular characteristics that can lead to good golf ball properties. However, one material generally cannot optimize all of the important properties of a golf ball. Properties such as feel, spin rate, resilience, and durability all are of importance, but improvement of one of these properties by use of a particular material often can lead to worsening of another. For example, ideally, a golf ball cover should have low hardness, high spin rate, and good feel, without sacrificing ball speed, distance, or durability. Such a cover would be difficult to make using only an ionomer resin having a high flexural modulus, because the resulting cover, while having good distance and durability, also will have poor feel and low spin rate, leading to reduced controllability of the ball.
To try to improve golf ball properties, some of the materials discussed above can be blended to produce golf ball cores, intermediate layers, or cover layers. As discussed above, ideally a golf ball cover should provide a proper spin rate and good feel, without sacrificing the ball's distance and durability. Therefore, an ionomer having a high flexural modulus often is combined in a cover composition with an ionomer or elastomer having a low flexural modulus. The resulting intermediate-modulus blend will have acceptable hardness, spin, and durability. In addition to the above materials, golf ball compositions also can include various fillers, fibers, colorants, and processing aids to impart additional desirable mechanical or cosmetic properties to the golf ball. An example of use of fibers in an intermediate layer of a golf ball is described in U.S. Pat. No. 6,012,991 to Kim et al. The fiber material described in that patent is added to intermediate layer compositions to enhance their hardness.
However, even with blending of materials to improve properties, use of the materials discussed above is not completely satisfactory. Improving one characteristic can lead to worsening another. For example, blending an ionomer having a high flexural modulus with an ionomer having a low flexural modulus can lead to reduced resilience and durability compared to use of the high-modulus ionomer alone. In general, it is difficult to make a material for an intermediate or cover layer for a golf ball that has low hardness, good feel, high speed, high resilience, and good shear durability. Similar difficulties exist in optimizing ball core properties.
In view of the above, it is apparent that materials are needed for use in making golf balls that allow the optimization of many ball performance properties without the worsening of other properties. The material also preferably should provide little or no added processing difficulties. The present invention fulfills this need and other needs, and provides further related advantages.